Longtime Paul Manafort protege and former deputy chairman for Donald Trump’s presidential campaign Rick Gates testified under oath on Monday that he helped his former mentor file false tax forms, in what has become the most dramatic moment of Manafort’s federal trial to date.

According to witnesses in the courtroom, Gates was asked specifically whether he’d been involved in criminal activity with Manafort, as well as if the two had expressly committed crimes together. Gates, who later helped to form the pro-Trump group America First Policies after Trump’s inauguration, testified that he and Manafort withheld more than a dozen foreign accounts from the government and worked to hide them “at Mr. Manafort’s direction.”

Gates, who pleaded guilty in February to charges of conspiring against the United States and lying to federal officials, is widely seen as the prosecution’s star witness in its case against Donald Trump’s onetime campaign manager. Since pleading guilty, Gates has cooperated with prosecutors, culminating in Monday’s testimony against his onetime boss.

Manafort, who is accused of a host of financial crimes largely predating his time on the president’s 2016 campaign team, reportedly spent the time during Gates testimony simply glaring at his former colleague.

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Manafort’s legal team, for their part, has worked to cast Gates as the sole perpetrator of any crimes during their time together. Gates also admitted to stealing significant amounts of cash from Manafort over the course of their work together.

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Manafort and Gates were the first two people indicted by Special Counsel Robert Mueller as part of his investigation into Russian interference in the 2016 election. As part of his prosecutorial mandate, Mueller can also pursue any subsequent crimes uncovered in the course of his investigation.

This is a developing story and will be updated as new details are made available.